Method and device for controlled straightening and cooling of a wide metal strip, especially a steel strip or sheet metal, running out of a hot rolled strip rolling mill

ABSTRACT

A method and device for controlled straightening and cooling of a wide metal strip, especially a steel strip ( 1 ) or sheet metal, running out of a hot rolled strip rolling mill, using pinching rollers ( 5 ) arranged in the moving direction of the strip ( 2 ) behind vertical double rollers ( 3,4 ), said pinching rollers producing a tensile stress ( 6 ) acting in a longitudinal direction. According to the invention, the range of use of conventional sheet metal cooling systems can be extended to obtain a more even surface of said steel strip ( 1 ) with an increased cooling effect. This is achieved by displacing the metal strip ( 1 ) or sheet metal between a pre-straightening machine ( 7 ) and splash cooling facility ( 8 ) in defined conditions of tensile stress ( 6 ) by adjusting the tensile stress ( 6 ) and by cooling said strip or sheet metal inside said splash cooling facility ( 8 ) between successive pairs ( 5   a ) of pinching rollers and by additionally controlling the tensile stress ( 6 ).

The invention concerns a method and device for controlled straighteningand cooling of wide metal strip, especially steel strip or sheet,running out of a hot-rolled strip mill, with the use of pinch rolls,which are installed after (in the direction of strip flow) verticaldouble rolls and produce a tensile stress that acts in the longitudinaldirection.

The method and device described above are basically known from DE 33 13024 A1, which discloses a method and a device for quenching passingsteel sheet, especially heavy and medium sheet, with simultaneouscontrolled straightening, in which the sheet, before being quenched, isstraightened in a plane while hot by at least two vertical double-rollsarranged one behind the other. The sheet is then drawn through aquenching installation under controlled tensile stress by at least onedouble roll. This method is designed to quench passing steel sheetexactly to the predetermined mechanical properties without thedevelopment of warpage or distortion and at the same to reduce roughnessthat is already present. Despite these measures, inadequate flatness ofthe sheet continues to occur due to inadequate application of tensilestress, unfavorable cooling, and other unfavorable control measures. Thecooling devices used in practice, which consist essentially of nozzleunits or, for the aftercooling, of laminar water curtains, cannot reallybe improved any further. In principle, a U-tube is used for laminarcooling units. Of course, the highest attainable cooling intensities areobtained with nozzles and suitably selected parameters. The poorautomatic control and regulation behavior is a disadvantage in generaland at low flow rates in particular.

The objective of the invention is to extend the range of application ofconventional sheet cooling systems to obtain greater flatness of thesheet with an increased cooling effect.

In accordance with the invention, this objective is achieved by movingthe metal strip or sheet between a pre-straightening machine and asplash cooling installation under defined conditions of tensile stressby adjustment of the tensile stress and by cooling the metal strip orsheet inside the splash cooling installation between successive pairs ofpinch rolls and additionally controlling the tensile stress. Thisdirectly produces plastic deformation before the metal strip or sheetpasses into the cooling installation. The plastic deformation improvesthe flatness of the entering metal strip or sheet. Another advantage isgained by the arrangement of the straightening process immediatelybefore the cooling, which prevents renewed buildup of internal stressesand deformations due to the nonuniform cooling on the surface of themetal strip or sheet.

The cooling effect is practically limited by the induction of stressesand deformations associated with these stresses in the sheet due to thenonuniform cooling effect of the cooling medium (water) on the surfaceof the sheet. This (unfavorable) effect occurs especially with the useof a relatively high cooling intensity on sheets that are thin and atthe same time wide. The critical product of a plant is defined by asheet with the least thickness at the maximum width and at the same timehigh intensity of the cooling. The intensity of the cooling is definedby a high temperature difference between the initial and final coolingtemperature using a short cooling time. In practice, pinch rolls areintended to hinder this deformation effect. However, the effect of thisform of compensation is limited. The arrangement of thepre-straightening machine before the cooling installation in accordancewith the invention brings about plastic deformation and reduces entrydeformations and stresses directly before the cooling installation.

The deformations on entry into the cooling installation can be combatedespecially effectively by adjusting the tensile stress by switching thespeed to the respective next or last supporting pinch roll withprogressive passage of the metal strip or sheet.

In one embodiment, the pinch rolls are set with a spacing such that acooling unit is formed between two rolls, depending on the spacing.

In a refinement of the invention, this spacing can be adjusted to ahalf-length of the resulting characteristic form for a strip or sheetwith the least thickness, at maximum width and high cooling intensity.

The device for the controlled straightening and cooling of a wide metalstrip, especially a steel strip or sheet, running out of a hot-rolledstrip mill is equipped with vertical pairs of double rolls and pairs ofpinch rolls which follow them (in the direction of strip flow), betweeneach of which a cooling unit is installed.

In accordance with the invention, the stated objective is achieved witha device of this type by installing a pre-straightening machine beforethe pairs of pinch rolls with the cooling units, between which acontrolled tensile stress can be produced. This makes it possible totransmit a tensile force even before the cooling, so that the magnitudeof the deformations that are present is already reduced here. An evenmore exact determination of the tensile stresses inside the coolingunits then occurs.

In accordance with other features, the cooling units are each designedfor a high and an intermediate cooling intensity.

The effect of the cooling can be increased still further by providingthe cooling unit with finely adjustable cooling elements at the outletof the metal strip.

The cooling can be further developed in such a way that the cooling unitis designed for a high and/or an intermediate cooling intensity inaccordance with a predetermined product spectrum.

The invention can be further improved by also making it possible toproduce tensile stresses between the last, supporting pinch rolls or thestraightening rolls at the trailing end of the metal strip or sheet andthe pinch rolls or the straightening rolls at the leading end of themetal strip or sheet.

The embodiments of the invention illustrated in the drawings areexplained in greater detail below.

FIG. 1 shows a system diagram of the cooling with pinch rolls and of thedrive control with the tensile stresses produced.

FIG. 2A shows a perspective view of the steel strip or sheet withdistortions in the form of center waviness.

FIG. 2B shows a side view of two pairs of pinch rolls arranged with acertain spacing distance.

FIG. 3 shows the arrangement of the cooling devices for the givencooling intensity.

FIG. 4 shows a view of a total installation for straightening andcooling.

The metal strip, especially the steel strip 1 or sheet, is conveyed by atensile stress 6 acting in the longitudinal direction (strip flowdirection 2) for controlled cooling and straightening with the use ofpinch rolls 5 arranged after (in the direction of strip flow 2)vertical, double rolls 3 and 4. In the process, the metal strip 1 ismoved between a pre-straightening machine 7 and a splash coolinginstallation 8 under defined conditions of tensile stress 6 byadjustment of the tensile stress (by the straightening roll rotationaldrive motors). The metal strip 1 is then cooled inside the splashcooling installation between successive pairs of pinch rolls 5 a, andthe tensile stress is additionally controlled. The splash coolinginstallation 8 comprises cooling units 9 installed between each twopairs of pinch rolls 5 a. (FIG. 1). While the metal strip 1 is enteringat roller table speed 1, the tensile stress 6 is controlled via asuitable torque 11 by a feedback control system 12, which is designated(A) action of the pinch rolls 5 (i) to (i+nc). As soon as the trailingend 1 a of the strip passes through, which is designated (B) action ofthe pinch rolls 5 (i) to (i+nc), suitable tensile stress is transmittedto the trailing end 1 a of the strip, and only the comparison speed isstill measured beyond the end 1 a of the strip without using thefeedback control system 12. Both for the trailing end 1 a of the stripand the leading end 1 b of the strip, the tensile stress 6 is adjustedby switching the speed to the respective next or last supporting pinchroll 5 with progressive passage of the metal strip (see FIG. 1).

The pinch rolls 5 are installed with a certain spacing 13, such that thecooling unit 9 is formed between two pinch rolls 5, depending on thespacing 13. For example, the spacing 13 can be based on a half-length 14of the resulting wavelength 15 for a metal strip with the leastthickness, at maximum width and high cooling intensity. The typicalwavelength 15 develops in the steel strip 11 (FIG. 2A).

The cooling units are each designed for a high or an intermediatecooling intensity (FIG. 2B), such that the spacing 13 corresponds to thehalf wavelength 15 (cf. FIG. 2B). The behavior of the wave 16 is clearlyshown by the dotted lines 17 in FIGS. 2A and 2B.

FIG. 3 shows the cooling effect of the different individual types ofcooling devices 9. In the graph of temperature as a function of time,the solid curve represents the behavior of the temperature 20 in thecenter of the strip or sheet, and the broken curve represents thesurface temperature 21. Splash cooling units 8 a are used in thesections (1). Cooling units 9, which consist of U-tube units 22, areprovided at the outlet 18 of the steel strip 1.

On the basis of the temperature ΔT-center (1), the temperature in thecenter of the steel strip 1 or the temperature ΔT-center (2) is reached.

The temperature ΔT-surface is reached while still in the zone of thesplash cooling units 8 a. When the U-tube units 22 are used, thenecessary cooling stop temperature is attained in small increments bythe U-tube units 22. Martensite formation by surface subcooling isprevented above the temperature T-martensite.

Tensile stresses 6 can be produced in the same procedure between thelast supporting pinch rolls 5 or straightening rolls 23 at the trailingend 1 a of the metal strip 1 or sheet and the pinch rolls 5 or thestraightening rolls 24 at the leading end 1 b of the metal strip 1.

FIG. 4 shows a complete installation, in which, in the direction ofstrip flow 2, the pre-straightening machine 7 is followed by the splashcooling installation 8 with the pairs of pinch rolls 5 a and the coolingunits 9 and then by a laminar cooling installation 25.

List of Reference Numbers

-   1 metal strip, steel strip or sheet-   1 a trailing end of the strip-   1 b leading end of the strip-   2 direction of strip flow-   3 double roll-   4 double roll-   5 pinch roll-   5 a pair of pinch rolls-   6 tensile stress.-   7 pre-straightening machine-   8 splash cooling-   8 a splash cooling units-   9 cooling unit-   10 roller table speed-   11 torque-   12 feedback control system-   13 spacing-   14 half-length-   15 resulting wavelength-   16 wave behavior-   17 dotted line-   18 outlet-   19 cooling element-   20 strip or sheet center-   21 surface temperature-   22 U-tube units-   23 straightening roll-   24 straightening roll-   25 laminar cooling installation

1. Method for controlled straightening and cooling of wide metal strip,especially steel strip or sheet, running out of a hot-rolled strip mill,with the use of pinch rolls, which are installed after (in the directionof strip flow) vertical double rolls and produce a tensile stress thatacts in the longitudinal direction, wherein the metal strip or sheet ismoved between a pre-straightening machine and a splash coolinginstallation under defined conditions of tensile stress by adjustment ofthe tensile stress, and that the metal strip or sheet is cooled insidethe splash cooling installation between successive pairs of pinch rolls,and the tensile stress is additionally controlled.
 2. Method inaccordance with claim 1, wherein the tensile stress is adjusted byswitching the speed to the respective next or last supporting pinch rollwith progressive passage of the metal strip or sheet.
 3. Method inaccordance with claim 1, wherein the pinch rolls are set with a spacing,such that a cooling unit is formed between two rolls, depending on thespacing.
 4. Method in accordance with claim 3, wherein this spacing canbe adjusted to a half-length of the resulting characteristic form for astrip or sheet with the least thickness, at maximum width and highcooling intensity.
 5. Device for controlled straightening and cooling ofa wide metal strip, especially steel strip or sheet, running out of ahot-rolled strip mill, with vertical pairs of double rolls followed (inthe direction of strip flow) by pairs of pinch rolls, between each ofwhich a cooling unit is installed, wherein a pre-straightening machine(7) is installed before the pairs of pinch rolls (5 a) with the coolingunits (9), between which a controllable tensile stress can be produced.6. Device in accordance with claim 5, wherein the cooling units (9) areeach designed for a high and an intermediate cooling intensity. 7.Device in accordance with claim 5, wherein the cooling unit (9) isprovided with finely adjustable cooling elements (19) at the outlet (18)of the metal strip (1).
 8. Device in accordance with claim 5, whereinthe cooling unit (9) is designed for a high and/or an intermediatecooling intensity in accordance with a predetermined product spectrum.9. Device in accordance with claim 5, wherein tensile stresses (6) canalso be produced between the last, supporting pinch rolls (5) or thestraightening rolls (23) at the trailing end (1 a) of the metal strip(1) or sheet and the pinch rolls (5) or the straightening rolls (24) atthe leading end of the metal strip (1) or sheet.